The hot-bitumen-impregnated cellulose corrugated sheets are known and their dimensions are standardized, whether it is de facto or by normalization.
Given the cost of the material used for making these sheets, in particular the cellulose fibre and the bitumen, it is desirable to reduce the required quantity of material while having interesting mechanical characteristics, in particular as regards the sheet inertia. The quantity of material is directly linked to the volume of material.
Now, the applicant has noticed that the inertia depends, in decreasing order of importance, on the corrugation height, the corrugation pitch, the corrugation radius and the thickness of material, and that the potential other parameters can be neglected. The volume of material depends itself, in decreasing order of importance, on the thickness of material, the corrugation pitch, the corrugation height and the corrugation radius, and the potential other parameters can be neglected.
Due to the fact that the same parameters intervene in these two characteristics of inertia and volume of material, the applicant has produced a formula making it possible to determine values of corrugation height, corrugation pitch and thickness of material that allow obtaining particularly interesting characteristics in terms of inertia-to-volume ratio for a corrugated sheet. The corrugation radius having a low impact on the volume of material, it is not taken into account in the formula.
The produced formula is an inequality. Moreover, in order to generalize the application of the formula, the applicant has more particularly taken into account the inertia per width unit as well as the volume per width unit. By way of information and as shown in FIG. 1, the length L of a corrugated sheet extends parallel to the crests (or troughs) of the corrugations and the width I extends perpendicularly to the corrugations. The width is hence transverse with respect to the corrugations.
A corrugated sheet is known from the document “Onduline Easyline”, of the 1st of Jun. 2003, page 1-1, XP055176511, but the dimensional characteristics thereof do not respect the formula proposed. Documents about “AQUALINE” corrugated sheets, respectively XP055176518 and XP055176519, or the patent application WO94/18379, are also known. None of these documents discloses such a formula or exposes such a problem of optimization. Furthermore, the inertia depends on many parameters, both of the dimensional and the composition type, and it is not taught in these documents to select more specifically certain of the parameters for the indicated purpose and for obtaining the formula, or even to modify, still for that purpose, more particularly certain of the parameters of a known sheet rather that the other ones, and to obtain a corrugated sheet having dimensions satisfying the formula.